Applications requiring strict access control, such as medical record query, often require auditing of the query. The current typical design relies on server side logging. However, logging on server-side do not provide strict means of auditing, since the server can be tampered with attackers, and also anybody who has permission to write can modify the log. We propose a scheme using blockchain technology, as a request-response channel for a client-server system, to record both client request and server reply in an audi-table manner. We have implemented a proof-of-concept system on top of a publicly available blockchain testbed. By using a blockchain as a client-server request-response channel, the request-response sequence can be verified by anybody who has access to the blockchain, providing a way to implement audit log for strictly controlled resources.